The duck was quietly resting upside-down against the stomach of Ian Gereg, an aviculturist here at the Livingston Ripley Waterfowl Sanctuary. Dr. Brennan, a post-doctoral researcher at Yale University and the University of Sheffield, visits the sanctuary every two weeks to measure the phalluses of six species of ducks.
When she first visited in January, the phalluses were the size of rice grains. Now many of them are growing rapidly. The champion phallus from this Meller’s duck is a long, spiraling tentacle. Some ducks grow phalluses as long as their entire body. In the fall, the genitalia will disappear, only to reappear next spring.
The anatomy of ducks is especially bizarre considering that 97 percent of all bird species have no phallus at all. Most male birds just deliver their sperm through an opening. Dr. Brennan is investigating how this sexual wonder of the world came to be.
Part of the answer, she has discovered, has gone overlooked for decades. Male ducks may have such extreme genitals because the females do too. The birds are locked in an evolutionary struggle for reproductive success.
Dr. Brennan was oblivious to bird phalluses until 1999. While working in a Costa Rican forest, she observed a pair of birds called tinamous mating. “They became unattached, and I saw this huge thing hanging off of him,” she said. “I could not believe it. It became one of those questions I wrote down: why do these males have this huge phallus?”
A bird phallus is similar — but not identical — to a mammalian penis. Most of the time it remains invisible, curled up inside a bird’s body. During mating, however, it fills with lymphatic fluid and expands into a long, corkscrew shape. The bird’s sperm travels on the outside of the phallus, along a spiral-shaped groove, into the female bird.
To learn about this peculiar organ, Dr. Brennan decided she would have to make careful dissections of male tinamous. In 2005 she traveled to the University of Sheffield to learn the art of bird dissection from Tim Birkhead, an evolutionary biologist. Dr. Birkhead had her practice on some male ducks from a local farm.
Gazing at the enormous organs, she asked herself a question that apparently no one had asked before.
“So what does the female look like?” she said. “Obviously you can’t have something like that without some place to put it in. You need a garage to park the car.”
The lower oviduct (the equivalent of the vagina in birds) is typically a simple tube. But when Dr. Brennan dissected some female ducks, she discovered they had a radically different anatomy. “There were all these weird structures, these pockets and spirals,” she said.
Somehow, generations of biologists had never noticed this anatomy before. Pondering it, Dr. Brennan came to doubt the conventional explanation for how duck phalluses evolved.
In some species of ducks, a female bonds for a season with a male. But she is also harassed by other males that force her to mate. “It’s nasty business. Females are often killed or injured,” Dr. Brennan said.
Species with more forced mating tend to have longer phalluses. That link led some scientists to argue that the duck phallus was the result of males’ competing with one another to fertilize eggs.
“Basically, you get a bigger phallus to put your sperm in farther than the other males,” Dr. Brennan said.
Dr. Brennan realized that scientists had made this argument without looking at the female birds. Perhaps, she wondered, the two sexes were coevolving, with elaborate lower oviducts driving the evolution of long phalluses.
To test this idea, Dr. Brennan traveled to Alaska. Many species of waterfowl breed there, with a wide range of mating systems. Working with Kevin McCracken of the University of Alaska and his colleagues, she caught and dissected 16 species of ducks and geese, comparing the male and female anatomy.
If a male bird had a long phallus, the female tended to have a more elaborate lower oviduct. And if the male had a small phallus, the female tended to have a simple oviduct. “The correlation was incredibly tight,” Dr. Brennan said. “When you dissected one of the birds, it was really easy to predict what the other sex was going to look like.”
Dr. Brennan and her colleagues are publishing their study today in the journal PLOS One.
Dr. McCracken, who discovered the longest known bird phallus on an Argentine duck in 2001, is struck by the fact that it was a woman who discovered the complexity of female birds. “Maybe it’s the male bias we all have,” he said. “It’s just been out there, waiting to be discovered.”
Dr. Brennan argues that elaborate female duck anatomy evolves as a countermeasure against aggressive males. “Once they choose a male, they’re making the best possible choice, and that’s the male they want siring their offspring,” she said. “They don’t want the guy flying in from who knows where. It makes sense that they would develop a defense.”
Female ducks seem to be equipped to block the sperm of unwanted males. Their lower oviduct is spiraled like the male phallus, for example, but it turns in the opposite direction. Dr. Brennan suspects that the female ducks can force sperm into one of the pockets and then expel it. “It only makes sense as a barrier,” she said.
To support her argument, Dr. Brennan notes studies on some species that have found that forced matings make up about a third of all matings. Yet only 3 percent of the offspring are the result of forced matings. “To me, it means these females are successful with this strategy,” she said.
Dr. Brennan suspects that when the females of a species evolved better defenses, they drove the evolution of male phalluses. “The males have to step up to produce a longer or more flexible phallus,” she said.
Other scientists have documented a similar coevolution of genitals in flies and other invertebrates. But Dr. Brennan’s study is the clearest example of this arms race in vertebrates.
“It’s rare to find something so blatantly obvious in the female anatomy,” Dr. Brennan said. “I’m sure it’s going on in other vertebrates, but it’s probably going in ways that are more subtle and harder to figure out.”
To test her hypothesis, Dr. Brennan plans to team up with a biomechanics expert to build a transparent model of a female duck. She wants to see exactly what a duck phallus does during mating.
Dr. Brennan also hopes to find more clues by studying phalluses on living ducks. At the waterfowl sanctuary in Litchfield, she is spending the year tracking the growth and disappearance of phalluses in ducks and geese. Hardly anything is known about how the phallus waxes and wanes — not to mention why.
“It may be easier to regrow it than to keep it healthy,” Dr. Brennan said. “But those are some of the things I may be able to find out. When you’re doing something that so little is known about, you can’t really predict what’s going to happen.”
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